It can be said that a solar cell generating electricity directly from the sunlight is one of the most promising future energy generation systems because it generates clean energy safely. Most solar cells that have succeeded in commercialization to date are based on silicon materials. However, processes for producing silicon adequate for solar cells application require a large amount of investment in plant and equipment and maintenance and operation thereof. Thus, such solar cells based on silicon are disadvantageous in terms of cost efficiency. As a substitute for those cells, many attempts have been given to thin-film solar cells because they use a relatively low amount of raw materials and are applicable to flexible substrates due to their light weight. As a result, thin-film solar cells have increased rapidly in market share recently.
Among such thin-film solar cells, copper indium gallium selenium (also referred to as CIGS hereinafter) solar cells have a very high photoelectric conversion efficiency of 20% or higher. When considering the technical characteristics and development rate of thin-film solar cells, it is expected that the CIGS solar cells will predominate in commercialization of solar cells in the near future. In addition, solar cells based on copper zinc tin sulfur (also referred to as CZTS hereinafter) obtained by substituting indium and gallium in CIGS with zinc and tin, respectively, and substituting selenium partially or totally with sulfur have characteristics similar to those of CIGS-based solar cells and are significantly cost-effective compared to the CIGS-based solar cells because zinc and tin are quite cheaper than indium and gallium, respectively. Thus, the CZTS-based solar cells have been considered as one of the most promising solar cells.
In general, a CIGS- or CZTS-based thin film solar includes: a substrate; a molybdenum back electrode layer; a light-absorbing layer; a buffer layer; and a transparent electrode layer. Forming each layer with less defects as and sequentially superimposing each layer with less interfacial defects are crucial in order to obtain high photoelectric conversion efficiency in a thin-film solar cell. Among the several processes of thin-film solar cell fabrication, a selenization process is very important. Selenization is usually conducted at high temperature under inert or reductive atmosphere incorporating selenium vapor to induce either (or both) densification of a thin film of light-absorbing layer or (and) microstructure control such as grain growth and grain alignment (texturing). Thus, the selenization process affects the physical and optical properties of the light-absorbing layer and causes microstructural change, for example, transformation of the molybdenum back electrode layer to molybdenum diselenide (MoSe2). Since molybdenum diselenide has very low unit cell density as compared to metallic molybdenum, selenization of molybdenum provides great volumetric expansion by about 4 times, as experimentally observed. Naturally, this large volume expansion during selenization of molybdenum induces high compressive stress between the molybdenum diselenide layer and the remaining molybdenum layer, leading to a main cause of interlayer delamination (partial or complete spallation of the back electrode) and finally mechanical failure of solar cells. Moreover, since the toughness of a molybdenum diselenide phase is significantly lower than that of molybdenum, the selenization of molybdenum is more detrimental to mechanical stability of the resultant thin-film solar cells.
The above-mentioned interlayer delamination prohibits construction of a high-efficiency thin-film solar cell because of severe mechanical damage and increased interlayer electrical resistance. Therefore, it is required to control the transport of selenium vapor from the selenization atmosphere to the molybdenum back electrode through a light-absorbing layer during selenization in order to prevent or minimize formation of molybdenum diselenide layer.